It is well known that cardiac arrhythmias such as ventricular fibrillation may be controlled with devices such as implantable defibrillators. Many different types of defibrillation electrodes have been suggested over the years, as can be seen from the following examples. In this discussion, the term defibrillation electrode will refer to both the conductive portion of the lead that delivers current to tissues, and any supporting core material or matrix material required to hold the desired shape of the conductive portion. Also, no distinction will be made between cardioversion and defibrillation; both will be referred to as defibrillation.
U.S. Pat. No. 3,942,536 issued to Mirowski et al. discloses an intravascular bipolar catheter electrode system wherein each of two electrodes is composed of a plurality of spaced rings. As implanted, the first electrode is located within the right ventricle (RV) and the second electrode is located in the superior vena cava (SVC).
In U.S. Pat. No. 4,161,952 issued to Kinney et al., a catheter electrode has a coil of wound spring wire, with filler material beneath and between individual turns of coil such that only the outside of the wound wire is exposed to the patient's body. It is designed to reside in or about the heart.
Other types of transvenously placed leads are disclosed in U.S. Pat. No. 4,998,975 issued to Cohen et al. One lead is placed through the heart wall, and into the pericardial space, and another is placed endocardially in a conventional manner. Both leads are shown with several embodiments, with the examples of general electrode construction being to expose a section of the conductor coil, or to use ring electrodes similar to those used in conventional bipolar pacemaker leads.
Another lead system patent, U.S. Pat. No. 5,007,436 issued to Smits, describes electrodes of both J and straight configurations, for use in the RV, right atrium, great cardiac vein, or coronary sinus (CS). The fabrication methods suggested use close wound conductive coils mounted exterior to an elongated insulative sheath, or the method of Kinney et al.
Spiral shaped electrodes for endocardial, epicardial, or extrapericardial implantation are described in Heil, Jr. et al., U.S. Pat. No. 5,01 6,808, Fogarty et al., U.S. Pat. No. 4,860,769, and Hauser et al., U.S. Pat. No. 5,052,407. The electrodes of these patents use various construction techniques, including electrodeposition or vapor deposition onto a plastic tube, helically wound wire (round or ribbon, unifilar or multifilar, single or double helix) or conductive rings on a flexible insulating portion, and conductive screen wrapped around a tubular body.
In U.S. Pat. No. 5,439,485 to Mar et al., assigned to the assignee of the present application, small diameter helically wound coils are wound onto a flexible core. These "electrode coils" are partially encapsulated by a flexible matrix which holds them in their wrapped position about the core.
Commercially available transvenous defibrillation / pacing leads provide sensing through either "integrated" or "dedicated" (also known as "true") bipolar electrode configurations. FIGS. 1 and 2, respectively, show Prior Art integrated and dedicated leads. In the integrated configuration, the pacing tip electrode 44 and defibrillation electrode 20 serve as the bipolar pair for sensing. In the dedicated configuration, a sensing ring 38 near the pacing electrode 44 and electrically isolated from the defibrillation electrode 20 forms a bipolar pair with the pacing electrode 44. In the integrated configuration, the defibrillation electrode 20, serving as one electrode of the bipolar pair, is relatively large.
If the bipolar signal is considered as an algebraic sum of unipolar signals measured at each pole of the bipolar pair of electrodes with reference to a remote electrode, then the difference between the integrated and dedicated configurations described above can be considered. Since the pacing tip is in intimate contact with the electrically active endocardial tissue and is small, the signal transmitted by this electrode is of relatively large amplitude and small pulse duration. The depolarization-repolarization wavefront will be sensed as it passes the electrode over a short distance. The unipolar signal sensed by the defibrillation electrode with respect to a remote electrode is smaller in amplitude and broader in pulsewidth. The lower amplitude is to be expected due to the remote location of the electrode in the blood pool, possibly covered with a layer of electrically inactive cells. The broader pulsewidth arises from the larger size of this electrode. The depolarization-repolarization wavefront may be sensed by the defibrillation electrode for a longer period of time as it spreads through the septum and right ventricle. A unipolar signal from a lead having a dedicated sensing ring with reference to a remote electrode represents a case intermediate unipolar sensing from the tip electrode and unipolar sensing from the defibrillation electrode, in that the ring is still fairly remote from active tissue like the defibrillation electrode but is smaller and closer to the pacing tip. As a result, the sensing ring signal amplitude is smaller than that sensed by the pacing electrode, but the sensed pulsewidth is narrower than for the defibrillation electrode.
To compare a dedicated bipolar signal to an integrated bipolar signal, the sum of the unipolar tip electrode signal and the unipolar ring electrode signal can be compared with the sum of the unipolar tip electrode signal and the unipolar defibrillation electrode signal. Thus, the dedicated bipolar configuration may be expected to produce a sharper bipolar sensing signal than the integrated bipolar configuration.
A problem with the use of the dedicated sensing ring is that it forces the defibrillation electrode to be located further from the cardiac tissue, and to be made shorter, assuming the length is constrained by the length of the ventricular chamber (apex to tricuspid valve); both the change in location and in length raise the defibrillation threshold. The dedicated sensing ring also requires not only an additional electrode but also an additional conductor, which may add cost, complexity, and / or lead size, or necessitate the use of new conductor technology. It also adds stiffness to the end of the lead, increasing the potential for perforations and exit block. However, the advantage of localized sensing makes the dedicated sensing electrode desirable.
In pacing electrode technology, the geometric dimensions of the electrode are minimized to increase the current density in adjacent tissue thereby lowering the stimulation threshold. However, sensing impedance is governed by the capacitive double layer at the surface of the electrode. From this standpoint, the larger the surface area, the lower will be the sensing impedance. Typically, microscopic electrode surface area is increased, for example by providing a porous surface structure, to reduce the sensing impedance. This surface structure is often used to advantage in pacing tip electrodes to encourage viable cell ingrowth, thereby increasing sensing signal amplitudes and decreasing stimulation thresholds.